The present invention relates to petroleum refining and more precisely to processes for the hydroconversion of crude oils, heavy hydrocarbon fractions and petroleum bottoms.
The feedstock which is used in the process according to the present invention may be any high-boiling hydrocarbon oil, for example an oil boiling above 350.degree. C. The initial source of the oil may be any hydrocarbon reservoir of ancient origin, including, besides crude oil, such materials as shale oil or oily sands, or liquid hydrocarbons resulting from coal liquefaction.
Petroleum and the oil fractions are very complex mixtures comprising, in addition to hydrocarbons, various compounds, mainly containing sulfur, nitrogen, oxygen and metals. These compounds are present in variable amounts and nature, depending on the origin of the crude oil and the oil fractions. They usually constitute impurities detrimental to the quality of the oil products, for reasons of pollution, corrosion, odor and stability. Among the many methods proposed for their removal, the catalytic treatments in the presence of hydrogen are the most common.
This technique has the advantage to yield products of good quality from crude oils and residues having a high content of impurities.
The difficulties encountered in the treatment of these feedstocks relate mainly to the presence of asphaltenes and metals which, under insufficiently controlled conditions, lead to a deactivation of the catalysts.
The contaminating metallic agents may be present as oxides or sulfides; they are however usually present as organometal compounds, such as porphyrins and their derivatives. The most common metals are vanadium and nickel.
The asphaltenes are present as a colloidal suspension which may agglomerate and settle on the catalyst in the conditions of hydrorefining. Thus the fixed bed hydrotreatment of these charges does not give satisfactory results, the catalyst deactivating as it is fouled with coke and metals.
The ebulliated bed technique, as applied to heavy feed charges (FP No. 2,396,065 and No. 2,396,066), reduces by 1.5 times the catalyst consumption with respect to the prior fixed bed processes and increases by approximately 2.5 times the production of liquid products as compared to the processes operated with preliminary deasphalting of the initial charge. This type of process is satisfactory for converting the soluble organometallic compounds; it is however less efficient as concerns the asphaltenes. Also, when using supported catalysts, some abrasion of the equipment occurs.
Another technique which remedies these insufficiencies, by allowing a better accessibility of the asphaltenes of high molecular weight to the catalytic sites, is disclosed in many patents, such as the French Patent No. 1,373,253 or the U.S. Pat. No. 3,165,463.
This result is attained by using catalytically active metal compounds in extremely divided form. These metal compounds are selected from the groups IV, V, VI or from the iron group, and they are used as a colloidal suspension or as a solution in a solvent. When introduced in the feed charge, they are converted to sulfides and, as the hydrorefining treatment progresses, a slurry forms which contains the catalyst, asphaltenes and various metallic impurities.
This technique implies that the heavy hydrocarbons and the catalyst slurry are separated from the total product discharged from the reaction zone. This operation is performed by any appropriate means, for example by distillation followed with a separation of the catalyst slurry; the latter is recycled to be combined with a fresh hydrocarbon charge. A portion of this slurry has however been previously removed, as catalyst purge, and has been replaced with a substantially equivalent amount of fresh catalytic compound.
This is commonly effected by feeding the pre-heating furnace preceding the reactor with both the fresh hydrocarbon charge, the slurry of recycled catalyst and the fresh catalyst, as described, for example, in the U.S. Pat. Nos. 3,331,769; 3,617,503 and 3,622,498.
Irrespective of the technique adopted, a relatively fast fouling of the pre-heating furnace is observed, as well as a decrease in performance.
The object of the invention is to obviate these drawbacks.